Musannaf Ibn Abi Shayba 24:125

Yahya ibn Adam narrated to us, he said: Sharik narrated to us, from Muhammad ibn Abdullah al-Muradi, from Amr ibn Murrah, from Abdullah ibn Salamah, who said: While a poet was reciting satire against Muawiyah and Amr ibn al-As on the day of Siffin, and Ammar was saying, "The servitude is for two wicked ones," a man said to him, "Glory be to Allah! Do you say this while you are companions of the Messenger of Allah, may Allah bless him and grant him peace?" Ammar said to him, "If you wish to sit, then sit, and if you wish to go, then go."